Travelling wave tube



Dec. 13, 1955 P. M. LALLY TAL TRAVELLING WAVE TUBE Filed May 4, 1954 ORS IN DM/P ATTORNEY United States Patent O TRAVELLING WAVE TUBE Philip M. Lally, Levittown, and Lee F. Bmash, Fresh Meadows, N. Y., assignors to The Spett" Rand Corporation, a corporation of Delaware Application May 4, 1954, serial No. 427,414

s Claims. (ci. sis-3.5)

This invention relates to travelling wave tubes, and more particularly, is concerned with improved coupling means between the helix of the travelling wave tube and the input and output coaxial line sections.

l This invention is an improvement over the travelling wave tube described in copending application, Serial No. 426,362, filed April 29, 1954, in the name of Seymour B. Cohn. ln designing high performance travelling wave tubes as therein described, it has been found advantageous to bring the input and output coaxial lines out at the collector end of the travelling wave tube to avoid distortion of the focusing magnetic field in the critical beam input region of the tube, ln such an' arrangement the input and output coaxial lines extend within the vacuum envelope' of the tube parallel to the tube' axis. This necessitates a double right angle bend transition at the ends of the helix to the respective input and output' coaxial lines. The space in which the transitions between the helix and the coaxial lines can be made is necessarily limited both longitudinally and radially in order to conserve magnet weight and power as well as tube size.

It is the general obiect of this invention therefore to provide an improved coupling between a helix of a travelling wave tube and the output and input coaxial lines, where' the coaxial lines are closely spaced from and extend parallel to the central longitudinal axis of the tube.

Another object of the invention is to provide an improved transition between a coaxial line and the hel-ix of vthe travelling wave tube which is simple in construction and easy to assemble.

Another object of the invention is the provision of coupling means which exhibits matched coupling of microwave energy between the helix and coaxial input and output transmission lines over an extended frequency band.

These and other obiccts of the invention which will become apparent as the description proceeds are achieved in Va travelling wave tube having a helix and coaxial inpuf and output line sections. The coaxial line sections extend parallel to the helix within the envelope of the tube.

To provide a matched coupling between the input end of the helix and the input coaxial line sections, a horn member is positioned at the input end of the helix and coaxial therewith. A hat plate extends between the outer surface of the horn and the adjacent end of the outer conductor of the input coaxial line, the plate having a surface parallelA to the plane deiined by the longitudinal axis of the helix and the longitudinal axis of the associated coaxial line. A conductive strip having a helically wound portion connected to the input end of the helix and a straight portion extending through an opening in the wall of the horn connects the helix to the inner conductor of the coaxial line section. The straight portion of the strip extends at right angles to the axes of the helix and coaxial line but is parallel to the plate, whereby the plate and strip form a strip-above-a-ground plane transmission line section. The width of the Strip and spacing from the ,plate are such that this transmission section has the same Cil il characteristic impedance as the coaxial line. The other end of the helix is similarly coupled to the output coaxial line section.

For a better understanding of the invention, reference should be had to the accompanying drawing, wherein:

Fig. l is an elevational View, partly in section, of a travelling wave tube incorporating the features of the present invention;

Fig. 2 is a fragmentary sectional view of the output end of the travelling wave tube;

Fig'. 3 is a cross-sectional view taken substantially on the line 3-3 of Fig. l; and

Fig. 4 is a cross-sectional view taken substantially on the line 4-4 of Fig. 2.

Referring to Fig. l in particular, the travelling wave tube as there illustrated includes an evacuated envelope indicated generally at lil, which includes an elongated cylindrical shell 12. Supported within the envelope 1 0' is a lirst pole piece and focusing electrode 14. An electron gun assembly, a portion of which is shown and indicated at f6, directs a stream of electrons through an aperture 18 in the pole piece M- along the longitudinal axis of the travelling wave tube. Electrical connections to the electron gun assembly le are brought out through an insulating base 19 and connected to suitable pins 21.

At the opposite end of the travelling wave tube is a second pole piece 29 secured within the envelope 10, the pole piece 2l) having an aperture 22 therein for passage of the electron stream'. The aperture 22 has an enlarged portion 24 for receiving a collector electrode 26. The' co'llector electrode 26 is supported by an end plate 23 forming part of the evacuated envelope lt?. For ease of assembly, a collar 30 is provided to support the end plate 2S from the shell l2, all joints between the shell 12, collar 3d, and end plate 2S being brazed after assembly to render them vacuum tight. K

Positioned adjacent one face of the pole piece 14 is a metallic cylindrical member 32 which projects into the region dened by the cylindrical shell l2. The member 32 has a tapered opening therein which is axially aligned with the aperture i8 along the longitudinal axis of the travelling wave tube. The member 32 with its tapered aperture may be characterized as a horn having a throat portion 36 and a mouth portion 37. A similar horn member 3S is secured to the pole piece 2%, the horn member 3S having a flaring mouth portion 49 and a throat portion 42.

Extending along the longitudinal axis along which the stream of electrons from the gun i6 is directed is a wire helix 44. The helix is supported along its length by three ceramic rods 46 which extend between and in turn are supported by the horn members 32 and 38. The ends of the rods 46 are positioned in holes or bores accurately formed in the horn members 32 and 3S, as particularly shown in the cross-sectional views of Figs. 3 and 4.

To couple R. F. energy into the helix 44 in accordance with the improvements of the present invention, an input coaxial line section 4S is provided which extends parallel to the longitudinal axis of the helix 44 within the radial space between the horns 32 and 38 and the outer shell 12,. The input coaxial line section 48 extends through the pole piece 20 and out through the end plate 28, the outer conductor 50 being brazed or otherwise secured to the end plate 28 to provide a vacuum type joint therebetween.

To achieve matched coupling between the helix 44 and the coaxial line 48, it is desirable to provide a transition section within the horn 32 in conformance with the teaching of the above-mentioned copending application. This transition involves a conductive strip having a helically wound portion 54 within the horn 32. As pointed out in the above-mentioned application, by proper tapering of the width and by proper spacing between the helically wound strip conductor and the throat portion 36 of the horn, a smooth transition is provided between the helix mode of propagation and a TEM mode of propagation within the throat of the horn member.

A significant feature of the present invention is the means of coupling between the coaxial line 48 and the transmission line formed by the throat 36 of the horn 32 and the tapered strip conductor 54. This coupling is provided by a strip-above-a-grcund plane type of transmission line section. The throat end of the horn member 32 is provided with a slot 56 opposite the end of the coaxial line 48. A conductive plate 58 is secured to the outer surface of the horn 32 adjacent the slot 56, the plate 58 having a surface 60 which is coplanar with a surface 62 formed by the slot 56. One edge of the plate 58 is joined to the end of the outer conductor 50 of the coaxial line section 48. The strip conductor 54 is provided with a straight portion 64 which is brought out through the slot 56 and joined at the end thereof to the inner conductor 52 of the coaxial line 48. The

straight portion 64 of the strip conductor 54 extends parallel to the surfaces 60 and 62 and is closely spaced therefrom. Thus the straight portion 64 forms with the surfaces 60 and 62 a strip-above-a-ground plane type of transmission line section. A step 66 in the plate 58 provides clearance for the inner conductor 52 of the coaxial line 48.

To achieve the desired broad band match between the helix 44 and the coaxial line 48 it is desirable that the characteristic impedance of the strip-above-a-ground plane transmission line provided by the conductive strip both within the horn and along the straight portion 64 where it extends out of the horn be equal to the characteristic impedance of the coaxial line 48, which is preferably the standard value of 50 ohms.

Referring to Fig. 2, the output from the helix 44 is similarly arranged. An output coaxial line section, indicated generally at 70, has an outer conductor 72 and an inner conductor 74. The output coaxial line 70 extends through the pole piece 20 and through the end plate 28, the outer conductor 72 being brazed or otherwise secured to the end plate 28 to provide a vacuum seal therewith. A strip conductor couples the end of the helix 44 to the inner conductor 74. The strip conductor is provided with a helically wound portion 76 within the horn member 38 and a straight portion 80 extending out through a slot 82 in the end of the horn member 38. A plate 84 is secured to the outer surface adjacent the slot 82, the plate 84 having a surface 86 which is coplanar with a surface 88 provided by the slot 82. The plate 84 is sei cured along one edge to the end of the outer conductor 72 of the coaxial line section 70. Thus, a strip-above-aground plane transmission line section is formed by the strip conductor, which is matched to the coaxial line section 7 0.

Since the straight portion 64 of the strip conductor at the input end of the helix and the straight portion 80 of the strip conductor at the output end of the helix are not shielded, means is provided for preventing coupling within the shell 12 between the input and output ends of the helix. This is achieved by annular shorting discs 90 and 92 which are secured respectively to the horn members 32 and 38. To provide ease of assembly, the shielding discs 90 and 92- are provided with a plurality of spring fingers, indicated at 94 and 96, which make sliding contact with the outer shell 12.

A rod 98 is secured at its ends to the respective discs 90 and 92 by means of screws 100 and 102. The purpose of the rod 98 is to provide a structural link between the two horn members 32 and 38 so-that the helix structure can be built as a subassembly before being positioned within the shell 12.

Suitable means (not shown) such as a permanent magnet or electromagnetic coil, may be positioned outside the tube in a manner to utilize the pole pieces 14 and 20 for establishing a magnetic focusing field for the electron stream.

It will be seen from the above description that the various objects of the invention have been achieved by providing a travelling wave tube in which the input and output coaxial lines are brought out at one end with the entire assembly of the helix and input and output coaxial lines being conned within the envelope of the tube. The unique strip-above-a-ground plane transmission line coupling arrangement between the helix and the input and output coaxial lines provides matched coupling despite the space limitations and sharp corners necessarily involved. By bringing the coaxial lines out parallel to the axis of the tube within the magnetic coil, the magnetic iield in the input region of the tube remains undistorted. At the same time by the resulting compactness of design, the path lengths of the flux lines are kept to a minimum, so that maximum field strength in the region of the electron stream can be maintained. It will be appreciated that the invention lends itself particularly to use in high frequency, low power tubes where the helix wire and associated horns are quite small in size. In such a case a standard 50 ohm coaxial line is so much larger than the horn that it can not be joined directly to the horn as in the above-mentioned Cohn application. However, the invention is not to be considered as limited only to high frequency tubes of small dimensions.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A travelling wave tube comprising a helix transmission line, means for projecting a stream of electrons along the longitudinal axis of the helix, an evacuated envelope enclosing the helix and said means, the envelope including a cylindrical conductive shell surrounding the helix and coaxial therewith, a pair of circular horns positioned at opposite ends of the helix and coaxial therewith, each horn having a throat portion and a aring portion, the flaring portions being directed toward each other, the horns each having a radially extending opening in the throat portion thereof, input and output coaxial transmission lines extending parallel to the longitudinal axis of the helix within the cylindrical shell, the coaxial lines projecting out of the end of the tube remote from said electron projecting means and respectively terminating within the tube adjacent the opposite ends of the helixl a first strip conductor connecting the input end of the helix to the inner conductor of the input coaxial line,

the iirst strip conductor including a helically wound portion of tapered Width positioned within the horn and connected to the end of the helix, and a straight portion extending at right angles to the longitudinal axis of the helix through the opening in the throat of the horn and connected to the end of the inner conductor of the input coaxial line, a rst conductive plate having a broad surface parallel to the straight portion of the rst strip conductor and parallel to the plane defined by the longitudinal axes of the helix and input coaxial line, the plate being secured along one edge to the end of the outer conductor of the input coaxial line and being secured along an adjacent edge to the horn adjacent said opening in the throat portion, whereby the strip conductor and the plate form a strip-above-a-ground-plane transmission line section, a second strip conductor connecting the output end of the helix to the inner conductor of output coaxial line, the second strip conductor including a helically wound portion of tapered width positioned within the other of said horns and connected to the output end of the helix,

ateatro and a straight portion extending at right angles to the longitudinal axis of the helix through the opening in the throat of said other horn and connected to the end of the inner conductor of the output coaxial line, a second conductive plate having a broad surface parallel to the straight portion of the second strip conductor and parallel to the plane defined by the longitudinal axes of the helix and output coaxial line, the plate being secured along an edge to the end of the outer conductor of the output coaxial line and being secured along an adjacent edge to said other horn adjacent said opening in the throat portion, and annular shielding means extending radially between said horns and said shell to prevent coupling between said first and second strip conductors.

2. A travelling wave tube comprising a helix transmission line, means for projecting a stream of electrons along the longitudinal axis of the helix, an evacuated envelope enclosing the helix and said means, input and output coaxial transmission lines extending parallel to the longitudinal axis of the helix within the evacuated f envelope, the coaxial lines projecting out of the end of the tube remote from said electron projecting means and respectively terminating within the tube adjacent the opposite ends of the helix, a first strip conductor connecting the input end of the helix to the inner conductor of the input coaxial line, the first strip conductor including a helically wound portion connected to the end of the helix and forming an axial extension of the helix, and a straight portion extending at right angles to the longitudinal axis of the helix and connected to the end of the inner conductor of the input coaxial line, a first conductive plate having a broad surface parallel to the straight portion of the first strip conductor and parallel to the plane defined by the longitudinal axes of the helix and input coaxial line, the plate being secured along one edge to the end of the outer conductor of the input coaxial line, a first conductive member coaxially surrounding the input end of the helix and helically wound portion of the first strip conductor, said first plate being joined to said first member, a second strip conductor connecting the output end of the helix to the inner conductor of output coaxial line, the second strip conductor including a helically wound portion connected to the end of the helix and forming an axial extension of the helix, and a straight portion extending at right angles to the longitudinal axis of the helix and connected to the end of the inner conductor of the output coaxial line, a second conductive plate having a broad surface parallel to the straight portion of the second strip conductor and parallel to the plane defined by the longitudinal axes of the helix and output coaxial line, the plate being secured along an edge to the end of the outer conductor of the output coaxial line, a second conductive member coaxially surrounding the output end of the helix and helically wound portion of the second strip conductor, said second plate being joined to said second member, and annular shielding means extending radially between said members and said envelope to prevent coupling between said first and second strip conductors.

3. A travelling wave tube comprising a helix transmission line, means for projecting a stream of electrons along the longitudinal axis of the helix, an evacuated envelope enclosing the helix and said means, a horn member positioned at the input end of the helix and coaxial therewith, the horn member having a throat portion and a flaring portion and having a radially extending opening in the throat portion thereof, an input coaxial transmission line extending parallel to the longitudinal axis of the helix within the evacuated envelope, the coaxial line terminating within the tube adjacent the input end of the helix, a strip conductor connecting the input end of the helix to the inner conductor of the input coaxial line, the strip conductor including a helically wound portion of tapered width positioned within the horn and connected to the end of the helix and forming an axial extension of the helix, and a straight portion extending at rightmangles to the longitudinal axis of the helix through the opening" in the throat of the' horn and connected to the end of the inner conductor of the input coaxial line, a conductive plate having a broad surface parallel to the straight portion of the strip conductor and parallel to the plane defined by the longitudinal axes of the helix and input coaxial line, the plate being secured along one edge to the end of the outer conductor of the input coaxial line' and being secured along an adjacent edge to the horn adja cent said opening in the throat portion, whereby the Strip conductor and the plate form a strip-'above-a-grondplane transmission line section, and annular shielding means extending radially between said horn and said nvelope to prevent coupling between said first strip cnductor and the output end of the helix.

4. A travelling wave tube comprising a helix transmission line, means for projecting a stream of electrons along the longitudinal axis of the helix, an input coaxial transmission line extending parallel to the longitudinal axis of the helix, the coaxial line terminating Within the tube adjacent .he input end of the helix, a strip conductor connecting the input end of the helix to the' inner conductor of the input coaxial line, the strip conductor including a helically Wound portion of tapered width connected to the end of the helix and forming an axial ex tension of the helix, and a straight' portion extending at right angles to the longitudinal axis of th helix and connected to the end of the inner conductor of the input coaxial line, a conductive plate having a broad surface parallel to the straight portion of the strip conductor and parallel to the plane defined by the longitudinal axes of the helix and input coaxial line, the plate being secured along one edge to the end of the outer conductor of the input coaxial line, a conductive member coaxially surrounding the input end of the helix and helically wound portion of the strip conductor, the plate being joined to said member, and annular shielding means extending radially from the outer surface of said horn member to prevent coupling between said first strip conductor and the output end of the helix.

5. A travelling wave tube comprising a helix transmission line, means for projecting a stream of electrons along the longitudinal axis of the helix, input and output coaxial transmission lines extending parallel to the longitudinal axis of the helix, the coaxial lines projecting out of the end ot' the tube remote from said electron projecting means and respectively terminating within the tube adjacent the opposite ends of the helix, a first strip conductor connecting the input end of the helix to the inner conductor of the input coaxial line, the first strip conductor including a straight portion extending at right angles to the longitudinal axis of the helix and connected to the end of the inner conductor of the input coaxial line, a first conductive plate having a broad surface parallel to the straight portion of the first strip conductor and parallel to the planeidefined by the longitudinal axes of the helix and input coaxial line, the plate being secured along one edge to the end of the outer conductor of the input coaxial line, a first conductive member coaxially surrounding the input end of the helix, said first plate being joined to said first member, a second strip conductor connecting the output end of the helix to the inner conductor of output coaxial line, the second strip conductor including a straight portion extending at right angles to the longitudinal axis of the helix and connected to the end of the inner conductor of the output coaxial line, a second conductive plate having a broad surface parallel to the straight portion of the second strip conductor and parallel to the plane defined by the longitudinal axes of the helix and output coaxial line, the plate being secured along an edge to the end of the outer conductor of the output coaxial line, and a second conductive member coaxially surrounding 7 the output end of the helix, said second plate being joined to said second member.

6. A travelling wave tube comprising a helix transmission line, means for projecting a stream of electrons along the longitudinal axis of the helix= a coaxial transmission line extending parallel to the longitudinal axis of the helix, the coaxial line terminating within the tube adjacent the end of the helix, a strip conductor connecting the end of the helix to the inner conductor of the coaxial line, the strip conductor including a straight portion extending substantially at right angles to the longitudinal axis of the helix and connected to the end of the inner conductor of the coaxial line, and a conductive plate having a broad surface parallel to the straight portion of the strip conductor and parallel to the plane defined by the longitudinal axes of the helix and coaxial line, the plate being secured along one edge to the end of the outer conductor of the coaxial line.

' 7. Apparatus for coupling one end of a helix transmission line to an adjacent end of a coaxial transmission line where the helix and coaxial lines are parallel and closely spaced, said apparatus comprising a strip conductor connecting the input end of the helix to the inner conductor of the input coaxial line, the strip conductor including a helically wound portion of tapered width connected to the end of the helix and forming an axial extension of the helix, and a straight portion extending at right angles to the longitudinal axis of the helix and connected to the end of the inner conductor of the input coaxial line, a conductive plate having a broad surface parallel to the straight portion of the strip conductor and parallel to the plane defined by the longitudinal axes of the helix and input coaxial line, the plate being secured along one edge to the end of the outer conductor of the input coaxial line, and a conductive member coaxially surrounding the input end of the helix and helically Wound portion of the strip conductor, the plate being joined to said member.

8. Apparatus for coupling one end of a helix transmission line to an adjacent end of the coaxial transmission line where the helix and coaxial lines are parallel and closely spaced, said apparatus comprising a strip conductor connecting the end of the helix to the inner conductor of the coaxial line, the strip conductor including a straight portion extending substantially at right angles to the longitudinal axis of the helix and connected to the end of the inner conductor of the coaxial line, and a conductive plate having a broad surface parallel to the straight portion of the strip conductor and parallel to the plane defined by the longitudinal axes of the helix and coaxial line, the plate being secured along one edge to the end of the outer conductor of the coaxial line.

No references cited. 

